The function of mammalian renal medulla is investigated with special emphasis on the role of renal pelves in exchanging solutes between pelvic urine and renal parenchyma. The extensions of the renal pelvis, the fornices, are studied by electronmicroscopy. In the inner medulla interstitial spaces which exchange solutes with the pelvic urine are studied using micropuncture techniques. Cell volume regulation in the inner medulla and in tissues of eurihaline fish are studied in vivo.